1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier or a laser beam printer, that employs an electrostatic recording system or an electrophotographic system.
2. Related Background Art
A two-component developing method that uses, as a developer, a mixture of a non-magnetic toner and a magnetic carrier has been widely employed for conventional electrophotographic image forming apparatuses, especially image forming apparatuses for performing chromatic color image forming. Compared with other currently proposed developing methods, the two-component developing method is superior in the stability of image quality and the durability of the apparatus. However, since as image forming is performed only toner is consumed, the supply of an appropriate toner must be replenished, as needed, and the toner density (the ratio of the weight of toner to the total weight of a developer) must be adjusted within an appropriate range. For the stabilization of image quality, it is extremely important that the toner density be adjusted within an appropriate range, and to achieve that objective, various methods have been proposed and put to practical use.
For example, a photodetecting method, an inductance detecting method, a patch detecting method, and a video counting method have been proposed and practically employed.
Above all, the patch detecting method is a method whereby the density of a reference toner image (hereinafter referred to as a patch image) formed on a photosensitive member is read using a light source, provided opposite the surface of the toner image, and a sensor, for receiving light reflected by the surface; and whereby, to adjust toner density, toner is replenished based on a value output by the sensor. Therefore, since a sensor need not be provided for each developing device, and since a cost savings can thus be realized, the patch detecting method has been widely employed.
When an image forming apparatus is employed for an extended period of time, toner supplied from a toner storage unit is consumed, so that the amount of toner remaining must constantly be monitored, and a user must be requested to replenish the supply as necessary. Conventionally, residual toner amount detecting means of a piezoelectric type, an antenna type and a photodetecting type have been proposed and have been employed. Another method is a toner presence/absence detecting method using the patch detecting method. When, for example, the detection output for the density of a patch image is equal to or smaller than a predetermined value, or when the value of a patch image density obtained after a forced toner replenishment operation has been performed is less than a predetermined value, it is determined that there is an absence of toner (for example, Japanese Patent Application Laid-Open No. H5-66669). And since only one sensor is required, both for detecting the presence/absence of toner and for controlling toner density, a special sensor for monitoring the toner supply is not required, making this is a very superior, cost efficient method.
However, using this method, a determination is made merely as to whether a value detected for patch image density is greater or smaller than a predetermined value, and it is difficult to ascertain whether toner has actually been exhausted or whether a factor other than toner density is responsible for a reduction in the patch image density. To resolve this problem, instead of the above method that employs, during normal operation, a value detected and output for the patch image density, a method that determines whether a detected output, obtained following the performance of a forced toner replenishment operation, exceeds a predetermined value can be employed to determine the presence/absence of toner. With this method, so long as there is toner remaining in the toner storage unit, the density of a patch image is absolutely increased following the forced replenishment of toner. Therefore, regardless of the state of the developer, whether toner is present or absent can correctly be ascertained.
However, when a fixed value is employed as a threshold value for determining the presence/absence of toner, the following problem has arisen: Whereas for a patch detecting operation performed during a normal toner replenishment process a reference signal value is corrected in accordance with the conditions encountered during that specific situation, when a fixed threshold value is used during an operation performed to detect the presence/absence of toner, a patch image density signal fluctuates, depending on the current situation, and the difference between the threshold and the detected value may be too large, or too small, so that the presence/absence of toner cannot correctly be determined.
Furthermore, when the environment of the image forming apparatus is changed, the γ characteristics of a developer also fluctuate, and relative to the toner density the sensitivity of the patch image density signal is also changed. Further, since the γ characteristics of the developer also change in accordance with the accumulative use period for the developer, accordingly, the sensitivity of the patch image density signal relative to the toner density is changed. As is described above, when relative to the toner density the sensitivity of the patch image density signal is changed, the amount of change in the patch image density signal varies relative to the same change in the toner density. In this case also, when the threshold value is fixed, the presence/absence of toner cannot be correctly detected.